This invention relates to compositions of matter and methods of using them to inhibit unwanted polymerization reactions of ethylenically unsaturated monomers. Polymerization inhibitors are known to be useful in preventing or reducing the unwanted polymerization of ethylenically unsaturated monomers which are handled at various stages of chemical processes in their manufacture and usage. Particularly, this invention illustrates a composition of inhibitors and retarders and its use in unwanted polymerization inhibition in handling the ethylenically unsaturated monomers.
Two categories of compounds have been developed to prevent unwanted polymerization reactions, inhibitors and retarders. Inhibitors prevent polymerization reactions from occurring. Inhibitors however are consumed rapidly. In cases of emergency when for mechanical or other reasons more inhibitor cannot be added, previously added inhibitor will be rapidly consumed and the unwanted polymerization reactions will then rapidly recur. Retarders slow down the rate of polymerization reactions but are not as effective as inhibitors. Retarders however are usually not consumed as quickly so they are more reliable in cases of emergency.
Ethylenically unsaturated monomers are reactive by their nature and they tend to polymerize through a radical polymerization mechanism. Unwanted polymerization reactions often impose operational concerns or difficulties while handling the ethylenically unsaturated monomers because the polymer formation may result in fouling and potential shutdown of operation equipment. Such is the case with the manufacture, transportation and storage of the monomers. It is especially a serious operational problem when distillation operation is involved in the manufacture and recovery of ethylenically unsaturated monomers, in which elevated operational temperatures accelerate the polymerization reactions of the monomers. Acrylonitrile is used as an example illustrating the handling of ethylenically unsaturated monomers, but not limited to, the manufacturing process described below.
The manufacture of acrylonitrile typically comprises three stages: the reaction, the recovery, and the purification stages. In the reaction stage, propylene, oxygen or air and ammonia are fed to a fluidized catalytic reactor. In the reactor, propylene undergoes a catalytic ammoxidation reaction to form acrylonitrile by reaction with ammonia and oxygen at an elevated temperature. The resulting acrylonitrile-containing reactor effluent is then cooled down in an aqueous quench column. In the quench column, unreacted ammonia is removed as ammonium sulfate by neutralizing with sulfuric acid. In the recovery stage the quench column overhead acrylonitrile-containing effluent undergoes a water absorption process in an absorber column to capture acrylonitrile and to dispose volatile components in the stream. Then, the effluent proceeds to a recovery column in which acrylonitrile is recovered through the overhead of the recovery column as an azeotropic distillate with hydrogen cyanide and water. This overhead distillate is then passed on to the purification stage.
In the purification stage, there are three sorts of equipment that are conventionally used in this sequence. First, a Heads Column is used to recover hydrogen cyanide from the feed. Second, a Drying Column dehydrates the acrylonitrile. Third, a Product Column yields the acrylonitrile product. In the Heads Column, hydrogen cyanide is separated as an overhead distillate through a conventional distillation operation. The bottom of the Heads Column is sent to a decanter for water removal. The organic phase of this decanter is routed to the Drying Column for further dehydration through an azeotropic distillation operation. The Drying Column overhead distillate is recycled back to the Heads Column, and the Drying Column bottoms are delivered to Product Column. In the Product Column heavy and light impurities are removed and a commercial grade of acrylonitrile is obtained as product. As with typical distillation operations, heating and elevated temperatures are involved in the Heads, Drying and Product column operations.
As one of the ethylenically unsaturated monomers, acrylonitrile tends to polymerize, and the polymerization reaction intensifies at elevated temperatures. Polymerization of acrylonitrile is unwanted in acrylonitrile manufacture since the resulting polymer tends to precipitate out of process stream, depositing on process equipment surfaces, and impairs the operation of the equipment. Acrylonitrile polymerization related fouling is often an operational concern for manufacturing and processing acrylonitrile, and it is especially a serious problem in the recovery and purification stages in acrylonitrile manufacture. Polymerization inhibitors have to be used routinely by acrylonitrile producers to mitigate polymerization induced fouling in process equipment or to stabilize acrylonitrile during transportation and storage. Nitroxide stable free radicals and hydroquinone (HQ) have been used to address these polymerizations.
HQ by itself is a less than optimal solution for unwanted polymerizations. As a polymerization inhibitor, HQ is only partially effective in inhibiting these polymerization reactions. Furthermore, HQ is a toxic chemical with environmental concerns.
Nitroxide stable free radicals, nitroxide hydroxylamines, N,N′-dialkyl/aryl substituted hydroxylamines, phenolic antioxidants, phenylenediamines and phenothiazines are well known reagents in the prevention of unwanted polymerization of ethylenically unsaturated monomers. The use of nitroxide stable radicals as polymerization inhibitors for ethylenically unsaturated monomers is mentioned for example in U.S. Pat. No. 3,744,988 which discusses the use of nitroxides, such as HTMPO in the inhibition of unwanted polymerization and resultant fouling in acrylonitrile manufacture. U.S. Pat. No. 4,670,131 discloses the use of stable free radicals, such as nitroxide, to inhibit polymerization of olefinic compounds.
Nitroxides are generally known as the most effective inhibitors owing to their superior inhibiting capability. Kinetically they are capable of scavenging carbon-centered free radicals at a nearly diffusion-controlled rate which is several orders of magnitude faster than phenolic compounds. However, their kinetic superiority is not always advantageous under certain circumstances. One issue of concern is their fast consumption rate as inhibitors. Other issues of concern are their consumption through non-inhibition, and their unwanted reactions with process stream components or other inhibitor additives. As a result, high nitroxide inhibitor dosages are often required for a given inhibition efficacy thereby making their use economically unattractive or even infeasible. Even worse, their interference with other inhibitors often results in antagonism and outright ineffectiveness as inhibitors.
U.S. Pat. No. 5,290,888 discloses the use of a N-hydroxy substituted hindered amine, such as 1,4-dihydroxy-2,2,6,6-tertamethylpiperidine, to stabilize ethylenically unsaturated monomers or oligomers from premature polymerization. N-hydroxy substituted hindered amines are usually prepared by reducing the corresponding nitroxide stable free radicals with a reducing reagent. N-hydroxy substituted hindered amines are excellent hydrogen donor due to the weak NO—H bond in the compounds, and thus they are efficient antioxidants. As antioxidants they react with peroxide radicals easily, while they are converted to their corresponding nitroxide. In essence, each N-hydroxy substituted hindered amine is equivalent to one hydrogen donor and one nitroxide inhibitor when peroxyl radicals and carbon-centered radicals are co-present, which is an attractive incentive offered by N-hydroxy substituted hindered amines. However, N-hydroxy substituted hindered amines are not stable when exposed to an oxygen-containing environment, such as storage in open air, as they are easily and gradually oxidized back to their corresponding nitroxide radicals.
One attempt to improve nitroxide inhibitors has been through the combination with other additives. When effective, these combinations are generally attributed to the combination of the fast kinetics of nitroxides in scavenging carbon-centered radicals and the durable action of other inhibitors or retarders in quenching carbon-centered and/or peroxyl radicals. Chinese patent application 86-1-03840 discloses a combined use of HTMPO and MEHQ in inhibiting the premature polymerization of methacrylic acid and isobutyric acid and its esters, but this requires the presence of oxygen. U.S. Pat. No. 5,728,872 discloses a combination of a nitroxide stable free radical, such as HTMPO, with a dihetero-substituted benzene having at least one transferable hydrogen, such as MEHQ, in inhibiting the premature polymerization of acrylic acid during the distillation process, either with or without the presence of oxygen. U.S. Pat. No. 5,955,643 discusses the use of a combination of nitroxide and phenylenediamine to inhibit the premature polymerization of vinyl aromatic monomers, such as styrene. U.S. Pat. No. 6,337,426 discusses a combination use of phenylenediamine and nitroxide to inhibit the premature polymerization of reactive light olefins, such as butadiene. U.S. Pat. No. 6,447,649 teaches the combined use of nitroxides and aliphatic amines to inhibit premature polymerization of vinyl monomers under both process and storage conditions. Examples of aliphatic amines are ethylenediamine, butane-1,4-diamine and propylamine.
Published PCT Application WO1998014416A1 teaches the combined use of a nitroxide and an oxime compound to inhibit the premature polymerization of vinyl aromatic monomers, such as styrene. U.S. Pat. No. 6,525,146 teaches the combined use of a hindered or unhindered phenol and an additional component selected from a nitroxide, a hydroxylamine, or a second different hindered or unhindered phenol to inhibit the premature polymerization of a diene compound, such as butadiene. US Published Patent Application 2004/0132930 discusses the combined use of a nitroxide and a dialkylhydroxylamine as a short-stopper in an aqueous suspension polymerization of vinyl chloride or mixed with another vinyl monomer. U.S. Pat. No. 5,322,960 discloses the combined use of a nitroxide, phenol and phenothiazine compounds in the inhibition of premature polymerization of (meth)acrylic acid and their esters. U.S. Pat. No. 6,300,513 discloses the combined use of an N-oxyl compound, N-hydroxy-2,2,6,6-tetramethylpiperidine, and 2,2,6,6-tetramethylpiperidine to stabilize vinyl compounds during their transport and storage. N-oxyl compounds are effective stabilizers for vinyl compounds, but they are lost with time in vinyl compounds. The co-presence of all three in a vinyl compound significantly reduces the loss of N-oxyl compounds with time in the vinyl compound. As a result, while the prior art does teach that nitroxides can sometimes be combined with other inhibitors or retarders these combinations often suffer from compatibility and storage issues, and are only useful in a limited number of environmental conditions.
For at least these reasons, there is a clear utility and novelty in other effective methods and compositions utilizing nitroxides and other polymerization prevention reagents for inhibiting unwanted polymerizations. The art described in this section is not intended to constitute an admission that any patent, publication or other information referred to herein is “prior art” with respect to this invention, unless specifically designated as such. In addition, this section should not be construed to mean that a search has been made or that no other pertinent information as defined in 37 C.F.R. §1.56(a) exists. Any and all patents, patent applications, and other references cited by this application are hereby incorporated by reference in their entirety.